1. Field of the Invention
The present invention relates to a radio communication system in which first radio terminals transmit uplink user data to a base station through an enhanced dedicated physical data channel and the base station transmits, to the first radio terminals, transmission rate control data for controlling a transmission rate of the uplink user data, and also relates to a radio communication method and a base station.
2. Description of the Related Art
Heretofore, a radio communication system including a base station and a radio network controller has been known. In the radio communication system, the base station has a single or multiple cells, and a radio communication is performed between each of the cells and a plurality of radio terminals. The radio network controller controls a plurality of base stations, and assigns a radio resource to the plurality of the radio terminals. Note that such a technique (hereinafter referred to as first technique) is sometimes referred to as Release 99 (R99) and others.
For the purpose of improving the throughput and shortening the delay time, and others, there has recently been proposed a technique in which a base station assigns the radio resources for uplink user data transmitted from each of the radio terminals to the base station (network side). Note that such a technique (hereinafter referred to as a second technique) is sometimes referred to as the High Speed Uplink Packet Access (HSUPA), the Enhanced Uplink (EUL) and others.
Each of the cells functions as a serving cell or as a non-serving cell. A Transport Block Size (TBS) is determined based on the transmission rate (for example, a Serving Grant (SG)) of the uplink user data, and is controlled by transmission rate control data transmitted from the serving cell and the non-serving call. The transmission rate control data includes Absolute Grant (AG) for directly indicating the transmission rate and Relative Grant (RG) for relatively indicating the transmission rate (for example, see 3GPP TS25.321 Ver. 7.5.0).
Here, the uplink user data is transmitted to the base station from the radio terminals through an Enhanced Dedicated Physical Data Channel (E-DPDCH). The absolute transmission rate control data (AG) is transmitted from the radio base station to the radio terminals through an E-DCH Absolute Grant Channel (E-AGCH). The relative transmission rate control data (RG) is transmitted from the radio base station to the radio terminals through an E-DCH Relative Grant Channel (E-RGCH).
The serving cell transmits the absolute transmission rate control data (AG) and the relative transmission rate control data (RG) to the radio terminals. Meanwhile, the non-serving cell transmits, to the radio terminals, only the relative transmission rate control data (RG) without transmitting the absolute transmission rate control data (AG).
Meanwhile, the base station has the upper limit of the radio resource (maximum radio resource) assignable to the radio terminal. Here, the maximum radio resource is a total of transmission rates assignable to the radio terminal by the base station (maximum receiving transmission rate).
In the above-described second technique, the base station can transmit the absolute transmission rate control data (AG) or the relative transmission rate control data (RG) at each TTI (Transmission Time Interval). In other words, the transmission rate assigned to the radio terminal is variable at each TTI.
Accordingly, in a case where the base station receives a communication start request from a new radio terminal in a certain TTI when a total of transmission rates having already assigned to the radio terminals in a TTI is close to the maximum receiving transmission rate, the base station is not capable of assigning a transmission rate to the new radio terminal.
For example, assume a case where a transmission rate assigned to a single radio terminal is high in a certain TTI. In this case, the transmission rate for uplink user data transmitted by the single radio terminal is maintained, but a new communication start request received in the certain TTI is rejected.
As described above, when a transmission rate is assigned to a radio terminal in each TTI without considering a new communication start request, the number of radio terminals communicable with the base station is likely decreased.